1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus.
2. Related Art
The integral photography method (hereafter referred to as IP method) of displaying a large number of parallax images or the light ray reproduction method of recording a stereoscopic image by using any method and reproducing it as a stereoscopic image is known. It is supposed that an object is viewed with left and right eyes. When a point located at a short distance is viewed, an angle formed by the point and the left and right eyes is denoted by α. When a point located at a long distance is viewed, an angle formed by the point and the left and right eyes is denoted by β. The angles α and β vary depending upon the position relationship between the object and the viewer. The angle difference (α−β) is called binocular parallax. Human being is sensitive to the binocular parallax and is able to conduct stereoscopic viewing.
In recent years, development of stereoscopic image display apparatuses without glasses has been promoted. Many of them use the ordinary two-dimensional plane display device. Some optical plate is placed on the front or back of the plane display device, and the binocular parallax described above is utilized. Light rays can be made to appear to be projected from objects located several cm before and behind the plane display device when a viewer views the plane display device, by controlling angles of the light rays projected from the plane display device with the optical plate. This is because it has become possible to obtain an image which is high in definition to some degree even if light rays of the plane display device are distributed to several angles (called parallaxes), owing to implementation of the plane display device having a higher definition.
A three-dimensional image display method obtained by thus applying the IP method to a stereoscopic image display apparatus is called II (integral imaging) scheme. In the II scheme, light rays projected from one lens correspond to the number of element image groups, and the number is typically called number of parallaxes. In each lens, parallax light rays are projected in parallel. In the II scheme, the viewer views different images such as an image of one parallax, an image of two parallaxes, and an image of three parallaxes according to the position of the viewer or an angle of viewer's viewing.
Therefore, the viewer perceives a solid by parallax between the right eye and the left eye. In the case where a lenticular lens is used as the optical plate, there is a merit that the display is bright because the utilization efficiency of light is high as compared with the case where a slit is used. It is desirable that the spacing between the lens array and pixels is substantially equal to the focal length of the lens. By doing so, one pixel can be projected in one direction. The viewer can view a different parallax image according to the viewing angle.
If a solid is made to be reproduced in a position away from the display surface in the stereoscopic image display apparatus of the II scheme, then an assigned light ray flux spreads via an opening or a lens, resulting in a problem of abrupt falling of the resolution (see, for example, H. Hoshino, F. Okano, H. Isono and I. Yuyama, “Analysis of resolution limitation of integral photography,” J. Opt. Soc. Am, A15 (1998), pp. 2059-2065.
In a stereoscopic image display apparatus which makes it possible to view a stereoscopic image without glasses, image information of a plane display device disposed on the back surface of an optical plate is assigned to respective parallax images. Therefore, the resolution falls as compared with the plane display device disposed on the back surface of the optical plate.
On the other hand, increasing the number of the parallax images is effective for improving the quality of the three-dimensional image, because it is possible to increase the viewing angle at which a normal stereoscopic image (three-dimensional image) can be viewed or to increase the far-side limit of the three-dimensional image position or the nearside limit of the three-dimensional image position. It is necessary to attempt to raise the resolution of the plane display device in order to increase the resolution, viewing angle, the far-side limit of the three-dimensional image position, and the nearside limit of the three-dimensional image position. However, there are restrictions because of the manufacturing cost.
Therefore, a stereoscopic image display apparatus in which the resolution is made high by displaying an image at a speed which is twice the conventional speed in a time direction, i.e., conducting time division display on the plane display device provided on the back surface of the optical plate is known. (JP-A2004-198727 and JP-A 2006-189833). In the display apparatus described in JP-A 2004-198727, two sheets of STN (Super-Twisted Nematic) arranged cells sandwiched between sheet polarizers are provided on the front surface of a liquid crystal panel, and the difference (transmission and interception) in retardation in the left and right eye direction is time-divided and changed over. The display apparatus described in JP-A 2006-189833 includes a display element having left eye video information and right eye video information, a video separator which separates incident light into a left eye video and a right eye video, a polarization conversion switch which converts the polarization direction of the incident light with time, and a one-layer double refraction element which transmits or refracts light according to a polarization direction of light passed through the polarization conversion switch. The resolution is improved by shifting the video converted in polarization direction by the polarization conversion switch through the double refraction element.
When conducting time division display by using the technique described in the above-described documents, there are the following problems.
First, the display apparatus described in JP-A 2004-198727 has a problem that the screen becomes dark because the light utilization efficiency becomes 50% if a scheme of repeating the interception and transmission of light fast is used.
Secondly, in the display apparatus described in JP-A 2006-189833, the viewing range is maximized and a stereoscopic image which is easy to view is obtained by providing parallax images in the direction of the viewer from all points in the screen when the eyes of the viewer is within the viewing zone width. When the viewer views a stereoscopic image, the viewing angle viewed by the viewer changes according to the position in the horizontal direction on the stereoscopic image display apparatus. In the stereoscopic image display apparatus, therefore, parallax images should be distributed around the normal direction of the plane display device in the central part of the screen. At ends of the screen, parallax images should not be distributed around the normal direction of the plane display device. When a line is drawn from the screen ends, the parallax images should be distributed around an angle of the vector direction of the line. When the directions of the light ray are distributed with time division, therefore, it becomes necessary that the accurate projection angles of parallax images can be calculated. This becomes especially important as the stereoscopic image display apparatus becomes large in size.
If accurate control in the parallax direction is not exercised, then a problem of occurrence of moiré, which changes in brightness according to the position, is posed.
Thirdly, in the case where the liquid crystal display apparatus is used in the plane display device which displays an elemental image or a changeover unit, it becomes important that the response rate of the liquid crystal is as fast as correspond to the time division display. In other words, the scheme of displaying 60 frames per second is typical at the present time. For conducting the two-division display in the time direction, therefore, 120 frames must be displayed per second. As for the response rate of the liquid crystal, therefore, the response of the liquid crystal must complete during 1/120=8.3 ms. If the response rate of the liquid crystal is slow, however, a former screen and a latter screen in the two-division display overlap each other and parallax images on respective display images overlap in the time direction. Since false parallax images are displayed in a direction which is not the original parallax direction, display degradation of the stereoscopic image such as blurring or a twin image occurs.